Impact of Soft Segment Composition on Phase Separation and Crystallization of Multi-Block Thermoplastic Polyurethanes Based on Poly(butylene adipate) Diol and Polycaprolactone Diol

Abstract

In this work, we explore the influence of soft segment structure on the crystallinity and phase separation of semicrystalline multi-block thermoplastic polyurethanes (TPUs) based on poly(butylene adipate) diol, polycaprolactone diol, and their mixture. According to thermal and structural analyses, the crystal growth rate and degree of crystallinity decrease with an increase in the PCL/PBA ratio and reach a minimum at the equimolar composition of polyesters. A reduction in crystal phase content leads to an improvement in elastomeric behavior. TPU samples with high PCL content demonstrate enhanced crystallinity but a lower melting temperature compared to TPU with PBA crystals. Crystallization of TPU below room temperature results in an enhancement of total crystallinity and a change in the phase composition of the PBA block. The difference in semicrystalline morphology and crystallization kinetics can be explained by the efficiency of phase separation and the density of hydrogen bonding between soft and hard segments. Our findings show that the ratio of the two crystallizable polyesters, combined with the choice of crystallization temperature, allows for independent control over the melting temperature and the overall degree of crystallinity of the TPUs. This significantly impacts the mechanical characteristics of the materials. The effect of adding a second crystallizable polyester on the crystallization behavior, phase composition, and mechanical properties of TPU is discussed for the first time.

1. Introduction

Semicrystalline thermoplastic polyurethanes (TPUs) are a fascinating type of adaptive polymer that holds significance from both fundamental and practical perspectives [1,2,3,4,5]. TPUs are linear block-copolymers that consist of a soft segment (SS) of crystallizable polyester and urethane hard segments (HSs). Due to the thermodynamic incompatibility of hard and soft segments, a phase separation (microsegregation) occurs in TPUs after thermal treatment [6]. The hydrogen-bonded domains of hard segments form a permanent physical network, while the crystallization of the soft segments creates an additional temporal network. The presence of both permanent and temporary networks determines the mechanical, relaxation, thermal, and degradation properties of the adaptive TPU materials [7,8,9].

The microphase separation in the melt can significantly influence the crystallization processes and resulting morphology of semicrystalline TPUs [10,11]. Furthermore, TPUs made up of different polyesters tend to exhibit more complex structural behavior, including geometry-confined crystallization due to the mutual effect of two crystal populations [12,13,14].

Poly(butylene adipate) (PBA) is a common biodegradable aliphatic polyester with distinct polymorphism, characterized by two separate phases: the thermodynamically stable monoclinic α-phase and the metastable orthorhombic β-phase [15,16]. The ratio of α to β is influenced by various factors, such as crystallization temperature, duration of storage, presence of nucleation agents in the melt, cooling rate, and extent of hydrogen bonding [17,18]. In previous studies, we have demonstrated that phase separation efficiency in TPU block-copolymers can be used to adjust the phase composition of PBA, thereby strengthening or toughening blends of biodegradable polymers for biomedical applications. The softer component forms a second phase within the more brittle continuous phase, which may act as a stress concentrator, enabling a ductile yield mechanism and preventing brittle failure. The addition of a second polyester, such as polycaprolactone (PCL), can alter the morphology and microphase separation, as well as control the crystallization kinetics.

The study [12] emphasizes the significance of the composition of soft segments in determining the properties and microstructure of TPUs. By adjusting the proportion of various soft segment constituents, it is feasible to customize the thermal and mechanical characteristics of TPUs to suit specific applications. Furthermore, the research underscores the intricate interdependence between phase separation, crystallization kinetics, and mechanical properties in multi-block TPUs. These findings can be valuable in developing novel TPUs with enhanced performance and properties.

The mechanical properties of TPUs depend on the physical network formed by hydrogen bonds or small crystallites, which can be tailored by varying the composition of SS and adjusting the degree of crystallinity. The type or ratio of polyol and/or diisocyanate, molecular weight of polyols, and structure of chain extenders strongly influence the mechanical properties of adaptive materials [19]. Structural investigations conducted in situ during deformation highlight intricate morphological alterations attributed to the interplay between phase separation and crystallization behavior. The distinct crystallization kinetics of PBA and PCL blocks provide an additional way to fine-tune the thermoplastic properties and shape memory behavior of TPUs.

The study underscores the importance of understanding how composition, microstructure, and properties are interrelated in the development of new TPUs with improved performance. By manipulating these factors, TPUs can be utilized in various industries. The physical network formed by hydrogen bonds or small crystallites of the soft segment determines the mechanical properties of TPUs, which can be adjusted by varying the composition of the soft segment and degree of crystallinity. The mechanical properties, such as tensile strength, Young’s modulus, and elongation at break, are influenced by the type or ratio of polyol and/or diisocyanate, molecular weight of polyols, and structure of chain extenders. Previous research [19] has shown that altering the chemical nature of the polyol and diisocyanates can produce multi-block TPUs with a wide range of mechanical properties. In situ structural investigations during deformation have revealed complex morphological changes due to the interplay between phase separation and crystallization behavior. The distinct crystallization kinetics of PBA and PCL blocks offer an additional way to fine-tune the thermoplastic properties and shape memory behavior of TPUs.

In the present study, a series of TPUs with PBA and PCL as SS, all having the same HS content, were synthesized and characterized. The impact of soft segment composition on the microstructure and crystallization behavior of TPU was examined using Fourier transform infrared spectroscopy (FTIR), wide-angle X-ray scattering (WAXS), differential scanning calorimetry (DSC), and mechanical measurements. To the best of our knowledge, this is the first report on the influence of soft segment composition on crystallization kinetics and microphase segregation in multi-block TPUs that contain two crystallizable blocks with identical molecular weights. These findings provide deeper insights into the thermal and mechanical behavior of multi-block semicrystalline TPUs with closely matched melting temperatures for their soft segments.

2. Materials and Methods

2.1. Materials

Poly(butylene adipate) diol (PBA) (Huakai Resin Co., Ltd., Shandong, China, M_n = 2000 Da) and polycaprolactone diol (PCL) (Merck, Darmstadt, Germany, M_n = 2000 Da) were dried in a vacuum for 4 h at 80 °C. The hydroxyl group content determined by the chemical method [20] was 1.7 wt.%. 2,4-toluene diisocyanate (TDI) and 1,6-hexamethylene diisocyanate (HMDI) from Merck (Darmstadt, Germany) were distilled in vacuum at 50–55 °C/12 mm Hg and stored in sealed ampoules. Chain extender 1,4-butanediol (BD) from Merck (Darmstadt, Germany) was distilled over freshly-powdered calcium hydride under reduced pressure. Dibutyltin dilaurate catalyst purchased from Merck (Darmstadt, Germany) was used as received.

2.2. Synthesis of Multi-Block Thermoplastic Polyurethane (TPU)

Synthesis of multi-block copolymers was carried out by the three-reactor method developed by us earlier at room temperature (25 °C) [21]. Two crystallizing polydiols were used in different weight ratios PBA/PCL (in wt%) 0/100 for TPU-1(0/100), 50/50 for TPU-2(50/50), 80/20 for TPU-3(80/20), and 100/0 for TPU-4(100/0). Urethane-diol fragments based on aliphatic and aromatic diisocyanates and chain extender BD form the hard segment. In synthesized polymers, the PCL and PBA blocks are linked in different combinations with bulky aromatic TDI and more flexible linear HMDI (

Table 1. Chemical composition of studied TPUs.

N°	Sample	Polymer Composition		SS, %		Mass Fraction of Reagents, %			SS/HS	HS,%
		A	B	A	B	Poly Diol	Diisocyanate	Chain Extender
1	TPU-1(0/100)	-	PCL	0	100	69	23	8	2.2	31
2	TPU-2(50/50)	PBA	PCL	50	50	69	23	8	2.2	31
3	TPU-3(80/20)	PBA	PCL	80	20	69	23	8	2.2	31
4	TPU-4(100/0)	PBA	-	100	0	69	23	8	2.2	31

). Upon reaching the degree of conversion of NCO groups ~98%, the reaction mass was poured into a flat Teflon container and dried at 40 °C during a day until constant weight. The degree of conversion was controlled by disappearance of the absorption bands of isocyanate (υ_NCO = 2272 cm⁻¹) and hydroxyl (υ_OH = 3623 cm⁻¹) groups in the infrared spectra.

The hard segment content (HS, %) was calculated as follows:

$$\mathrm{HS}(\%)=\frac{\left(1+n\right) M\left(TDI+HMDI\right)+nM\left(\mathrm{BD}\right)}{\left(1+n\right) M\left(TDI+HMDI\right)+nM\left(\mathrm{BD}\right)+M\left(poly diol\right)}\times 100\%$$

where M(poly diol), M(TDI+HMDI) and M(BD) are the molecular weights of PBA and/or PCL; diisocyanate and chain extender, respectively; n is the number of moles of BD.

2.3. Characterization

The functional groups inherent to each polymer were investigated via Fourier transform infrared (FTIR) spectroscopy Bruker Appha (Bruker Daltonics GmbH & Co.KG, Bremen, Germany). The spectra were recorded on a Bruker Alpha spectrometer using a multiple attenuated total reflection (ATR) module under the following conditions: measurement range 4000–500 cm⁻¹, measurement step 2 cm⁻¹, and the number of scans per spectrum −56.

The thermal properties were analyzed by differential scanning calorimetry (DSC) using a DSC 30 (Mettler Toledo, Columbus, OH, USA) calorimeter at a heating/cooling rate of 10 °C/min under nitrogen atmosphere. The temperature and enthalpy of melting were calculated using the first scans from 23 to 100 °C. Glass transition temperature was measured during the second heating scan from −80 to 100 °C. In case of multiple endothermic events each melting peak was fitted with a Gaussian function. The ∆H and T_max were calculated as area and maximum of the corresponding Gaussian peak, respectively. T_onset is defined as the intersection of the tangent line at the half-width point with x-axis. The degree of crystallinity (χc) of PBA and PCL blocks was determined by Equation (1):

$$\mathsf{\chi }\mathrm{c} =\frac{\Delta Hm}{\Delta H° m}$$

(1)

where ∆H_m is the total melting enthalpy of PBA or PCL measured from DSC curves, ∆H°_m is the melting enthalpy of 100% crystalline PBA (95 J/g) and PCL (203 J/g), respectively. These values were established previously from DSC and X-ray diffraction data of the as-received polyols [22].

The crystallization kinetics of the TPUs were studied under isothermal conditions. A sample (10 ± 0.2 mg) was heated above the melting point of the soft segment (100 °C) and then rapidly cooled to the crystallization temperature of 25 °C or −5 °C. The crystallization was monitored by the appearance of an exothermic peak on a time-resolved DSC curve. A change in the degree of crystallinity χc(t) during the isothermal crystallization of PBA or PCL was determined from the ratio between the enthalpy of melting of the sample and the enthalpy of melting corresponding to a 100% of crystallinity for polydiols according to Equation (2):

$$\mathsf{\chi }\mathrm{c}\left(\mathrm{t}\right)=\frac{{{\displaystyle \int }}_{\mathrm{o}}^t\left(\frac{dH}{d\tau }\right)d\tau }{\Delta Hm}$$

(2)

where dH/dτ is the heat flow during crystallization, and $\Delta Hm$ denotes the equilibrium melting enthalpy of PBA and PCL, respectively.

A detailed study of crystallite growth during isothermal crystallization was conducted using the classical Avrami equation [23,24]:

$$1-{\mathsf{\chi }}_{\mathrm{c}}\left(\mathrm{t}\right)=\exp (-K{t}^n)$$

(3)

where n is the Avrami exponent, indicative of the nucleation mechanism and the geometry of growing crystals, and K is a constant characterizing both the nucleation rate and the rate of crystal growth.

The parameters n and K were determined from the dependence ${\mathsf{\chi }}_{\mathrm{c}}\left(\mathrm{t}\right)$ in Avrami coordinates (Equation (4)):

$$log(1-ln\left(1-{\mathsf{\chi }}_{\mathrm{c}}\left(\mathrm{t}\right)\right)=n·\log t+\log K$$

(4)

The crystallization half-time (t_50%) was calculated using the Avrami parameters as follows (Equation (5)):

$$t_{50\%}={\left(\frac{\mathit{ln}2}{K}\right)}^{1/n}$$

(5)

A DSC822 (Mettler-Toledo) thermogravimetric analyzer (TGA) was used to investigate the thermal degradation behavior of the samples. The analyses were performed from 25 to 550 °C at a heating rate of 10 °C/min under nitrogen flow (20 mL/min). The sample masses were ~5 mg. The experiments were conducted in an inert nitrogen atmosphere to prevent oxidation and potential secondary reactions during thermal degradation of the TPU samples.

Tensile tests were performed at room temperature on a Zwick TC-FR010TH tensile-testing machine at the drawing rate of 100 mm/min.

Small-angle X-ray scattering (SAXS) was measured on SAXS/WAXS diffractometer (Xenocs, Grenoble, France). The wavelength was 1.54 Å and sample-to-detector distance was 1600 mm. High-temperature measurements were performed with Linkam LTS 420 heating stage.

Wide-angle X-ray scattering (WAXS) analysis of the TPUs was carried out using the Kurchatov complex of synchrotron-neutron studies in Moscow, utilizing the BIOMUR apparatus with a wavelength λ of 1.44 Å. The exposure time was set at 180 s, and the distance between the sample and the detector was 165 mm. Two-dimensional diffraction patterns were captured using a Pilatus 1M detector. The modulus of the scattering vector (s) was calibrated with several diffraction orders of silver behenate and processed using a custom-built software environment. The degree of crystallinity (χ) was then calculated according to:

$$\mathsf{\chi }=\frac{{\displaystyle \int }{I}_{cr}\ast s^{2}ds}{{\displaystyle \int }\left(I_{cr}+I_{amorph}\right)\ast s^{2}ds}$$

(6)

where I_cr and I_amorph are areas under all crystalline reflections and amorphous halo on 1D diffractograms. The crystallite size (D) was calculated using the Scherrer equation:

$$D=\frac{0.9 \lambda }{\Delta s}$$

(7)

where Δs represents the full width at half maximum (FWHM) of the selected diffraction peak.

3. Results

The synthesized TPUs form a phase-separated morphology of the thermodynamically incompatible SS and HS. The schematic chemical structure of TPU is presented in our previous article [19].

3.1. Thermal Stability Analysis of the Synthesized TPUs via TGA

Thermogravimetric analysis (TGA) (Mettler Toledo, Columbus, OH, USA) of the synthesized thermoplastic polyurethanes (TPUs) reveals notable differences in thermal stability based on the composition of the soft segments, particularly the PBA/PCL ratio (Figure 1,

Table 2. Results of Thermogravimetric Analysis (TGA) measurements for selected TPUs conducted under nitrogen atmosphere.

Sample	PBA/PCL,%	DTGmax, °C	T(5%), °C	T(10%), °C	T(50%), °C	T(80%), °C	T(endset), °C	Residual Weight, %
PCL	-	370/417	302	318	367	396	463	-
TPU-1(0/100)	0/100	345/436	275	293	337	355	477	-
TPU-2(50/50)	50/50	351/394/447	288	311	365	403	550	3.6
TPU-3(80/20)	80/20	360/406/449	293	315	375	411	550	2.9
TPU-4(100/0)	100/0	361/407/449	294	320	379	413	550	4.2
PBA	-	413	316	360	407	398	439	-

). This evaluation utilized the temperature corresponding to a 5% mass loss (T5%), along with other defining temperatures associated with weight losses (10%, 50%, 80%, and endset). The residual weight at 550 °C served as an additional measure of thermal stability.

The TGA curves show superior thermal stability of the PBA diol in comparison to PCL diol of identical molecular weight (2000 Da). The decomposition of PBA revealed via the derivative thermogravimetry (DTG) curves, occurs singularly at the peak (DTGmax) of 413 °C, while the PCL’s decomposition process is biphasic. The TPU-1(0/100) sample, solely comprised of the PCL block, demonstrates a two-phase decomposition process. It is characterized by the absence of residual mass and possesses the lowest thermal stability in the series (T_5% = 275 °C). In contrast, TPUs with varied proportions of PBA and PCL, specifically, TPU-2(50/50), TPU-3(80/20), and TPU-4(100/0), exhibit a four-stage decomposition process with a residual mass of 3–4%.

Furthermore, the data show that as the percentage of PBA in the TPUs increases, their thermal stability improves. This suggests that the thermal degradation is influenced by the stability of the soft segment, and in these cases, is directly proportional to the PBA content.

3.2. Thermal Transitions and Crystallization Kinetics of TPUs

Variations in the thermal transition temperatures, as shown in Figure 2a,b and detailed in

Table 3. Thermal properties of TPUs with different polyol compositions annealed for 0.5 months.

Sample	PBA/PCL	Tg, °C	Tc, °C	Tonset, °C	Tmax, °C	∆Hm, J/g	ΧcPBA, %	ΧcPCL, %
TPU-1(0/100)	0/100	−51	25	38	48	21.4	-	10
		−51	−5	40	47	23.8	-	12
TPU-2(50/50)	50/50	−49	25	40/52	46/61	1.1/1.9	2	0.5
		−49	−5	42/54	48/64	17.9/0.9	1	13
TPU-3(80/20)	80/20	−49	25	39/49	43/54	2.7/19.2	20	1
		−49	−5	41/44	48/55	7.9/14.2	15	4
TPU-4(100/0)	100/0	−48	25	43	51	30.4	31	-
		−48	−5	42/49	49/55	5.8/17.6	25	-

, indicate that the structure of the soft segment influences the thermal properties of TPU samples. Figure 2a presents the DSC curves (the first heatings) of TPU samples that were crystallized at 25 °C for half a month. For TPU-1(0/100), an endothermic peak is observed with an onset temperature of 38 °C (Figure 2a, blue curve), which corresponds to the PCL crystals melting. In contrast, for TPU-4(100/0), there is a peak shift to 43 °C. This shift is attributed to the melting of PBA α crystals (Figure 2a, black curve) [2]. In the thermogram of TPU-2(50/50), which has an equal content of PBA and PCL, two subtle endothermic peaks at 40 and 52 °C can be discerned. This suggests a low overall degree of crystallinity and a gradual crystallization process for both PCL and PBA crystal phases (Figure 2a, red curve).

For TPU-3(80/20), two distinct melting peaks at 39 and 49 °C are observed (Figure 2a, green curve). The inclusion of a minor fraction of PCL blocks appears to reduce the overall crystallinity while preserving the melting temperature of both soft segments, as detailed in Table 3. The concurrent presence of the two soft segments complicates their crystallization, hindering the phase separation of PBA and PCL segments.

At −5 °C, the PCL crystal growth rate remains low, with only a slight increase in DSC peak intensity. The appearance of a second melting peak for TPU-4(100/0) corresponds to the formation of a metastable β-modification at a reduced crystallization temperature with minimal changes in the total degree of crystallinity. For the samples containing both PBA and PCL, crystallization at −5 °C accentuates independent crystallization of PBA and PCL. The DSC curve of TPU-2(50/50) reveals an intense peak at 42 °C due to the PCL crystals melting, and a shoulder at 54 °C, suggesting that PBA crystals’ nucleation might be impeded by the earlier formed PCL lamellae [22]. However, rapid crystallization of PCL results in a considerable elevation in total crystallinity compared to crystallization at room temperature (Table 3). For TPU-3(80/20) with a higher PBA content, the decrease in crystallization temperature has a smaller effect on total crystallinity. The increase in the relative intensity of the first peak at 41 °C and in the total crystallinity indicates the preferential crystallization of PCL, even at a high PBA/PCL ratio. The shift of the second melting peak onset to lower temperatures can be attributed to the formation of a β-polymorph of PBA. Consequently, crystallization conditions can be adjusted to control the crystalline phase fraction and the melting temperature. In previous studies, we observed a similar intricate thermal behavior in semicrystalline polyurethane–urea systems during extended storage [21]. The relative content of concurring polyols affects the total degree of crystallinity. However, one can see that the crystalline fraction of each polyester is not proportional to its content. Consequently, we have the mutual effect of a PBA and PCL crystal phase on the final morphology of the material.

The influence of the polyol composition of the TPU samples on the crystallization kinetics of the soft segment at 25 and −5 °C was studied by DSC. The crystallization isotherms and the sigmoidal crystallinity relationships for TPUs with various polyols are depicted in Figure 2b,c. For TPU-4(100/0) at 25 °C, the normalized isotherm reveals a wide exothermic peak linked to PBA crystallization, resulting in a half-crystallization time (t50%) of 22.5 min (see Figure 2c, black solid curve, Table S1). Conversely, PBA’s crystallization at −5 °C is more rapid than at 25 °C, with a crystallization half-time of 14.6 min (Figure 2b, black dashed curve). This aligns well with the heating scans displayed in Figure 2.

In TPU-1(0/100), the half-crystallization time of PCL at −5 °C is t_50% = 11.1 min (Figure 2b,c, blue dashed curve), comparable to that of PBA. This enhanced crystallization efficiency of PCL in TPUs that contain both diols can be attributed to the distinct phase separation behavior of the segments in the molten state.

Applying the Avrami equation (Figure 1S) allows further analysis of the TPUs’ thermal behavior for the crystallization temperatures of 25 °C and −5 °C [23,24,25]. The Avrami parameter n for TPU-4(100/0) and TPU-1(0/100) was identified as 2.3 and 2.5, respectively, suggesting three-dimensional spherulite growth with isothermal nucleation in line with Avrami’s theory [26]. These non-integer values point out diffusion limitations on segmental mobility at the crystal growth front. The Avrami parameter n was found to be unaffected by temperature and the soft segment type in TPUs with a solitary crystallizable segment. However, due to the low crystallization rate of TPU-1(0/100), TPU-2(50/50), and TPU-3(80/20) at room temperature, the kinetics of independent PBA and PCL segments’ crystallization in these TPUs could not be studied using DSC. Therefore, wide-angle X-ray scattering (WAXS) was employed to analyze the crystal phase composition of the TPUs. Modifying the soft segment’s chemical composition and crystallization temperature can tailor the crystallization kinetics of both segments.

3.3. Crystal Structure and Phase Composition of TPUs

In Figure 3a, X-ray diffractograms of the studied samples crystallized at 25 and −5 °C are presented. Phase composition, d-spacing of the most intense reflections, and crystal size are summarized in

Table 4. Structural parameters of the studied samples obtained by WAXS.

Tc, °C	ΧWAXS, %	PCL or β-PBA		α-PBA			D, nm
		Χ, %	d(110), Å	Χ, %	d(110), Å	d(020), Å
TPU-1(0/100)
25	12	12	4.18	-	-	-	14
−5	16	16	4.25	-	-	-	17
TPU-2(50/50)
25	6	-	-	-	4.01	3.87	-
−5	11	-	-	-	4.03	3.85	-
TPU-3(80/20)
25	12	-	-	12	4.09	3.96	20
−5	18	2	4.16	16	4.07	3.95	23 (PBA) 24 (PCL)
TPU-4(100/0)
25	17	-	-	25	4.09	3.97	19
−5	20	5	4.18	15	4.10	3.98	18 (α) 13 (β)

. TPU-1(0/100) shows crystalline reflections at s = 0.239 and 0.265 Å⁻¹ typical for the orthorhombic structure of PCL [27]. The decrease in crystallization temperature from 25 to −5°C results in a significant increase in the degree of crystallinity and crystal thickness (Figure 3a, blue curves).

For the TPU-2(50/50), a broad crystalline peak located at 0.248 Å⁻¹ is observed regardless of crystallization conditions. It consists of two overlapping peaks corresponding to the (110) reflections of the α-modifications of PBA and PCL. A small shoulder with d-spacing 3.87 Å related to (020) reflection of α-PBA is additional evidence of α-PBA crystal formation. Failure to separate the broad peaks does not allow for the estimation of crystallite size.

For the TPU-3(80/20), α-PBA phase formation at both temperatures is observed. However, after crystallization at −5 °C, the appearance of a small peak at 0.240 Å⁻¹ and a broad peak with a maximum at 0.272 Å⁻¹ indicates the presence of a crystal phase of PCL (Figure 3a, green curves).

The diffractogram of TPU-4(100/0) solely based on PBA crystallized at 25 °C reveals the formation of only α-modification of PBA with a high degree of crystallinity (χ = 17%). Crystallization at −5 °C shows the enhancement of crystallinity due to the formation of β-phase with thinner crystals (Figure 3a, black curves). In general, a decrease in crystallization temperature leads to the growth of PCL crystal fraction or formation of the β-phase of PBA, which is in agreement with the DSC results. The variation in crystallinity χ (from WAXS) as a function of PBA content (Figure 3b) for both crystallization temperatures has a symmetric form with a minimum of 50% of PBA because of the separated crystallization of PBA and PCL blocks. This allows independently for the varying degree of crystallinity and melting temperature, providing a very interesting option for the fabrication of materials with a shape memory effect. The difference in crystallinity measured by DSC and WAXS can be explained by the influence of the adjacent HS on the equilibrium melting enthalpy of SS. The dependence of phase composition on crystallization temperature can be associated with a balance between microsegregation of the blocks and nucleation rate. A detailed analysis of the phase separation of SS and HS in TPUs was performed by SAXS and the FTIR technique.

3.4. Study of Phase Separation in TPUs Using FTIR Analysis

The phase separation of HS and SS in the studied TPUs was proved by SAXS (Figure S2). Samples crystallized at 25 °C show an intensive peak related to semicrystalline lamella morphology with a long period of 18 nm. After heating the above soft block melting points (up to 80 °C), the long period increases to 21 nm with a significant drop in intensity, which can be explained by preserving the phase-separated amorphous morphology.

Figure 4 shows typical FTIR spectra of the TPU films within the absorption range of 4000–900 cm⁻¹. The absorption bands associated with the stretching and bending vibrations of the NH group (υNH ~3500 to 3200 cm⁻¹ and δNH ~1580 to 1490 cm⁻¹) and the stretching vibrations of the carbonyl group (υC=O ~1800 to 1640 cm⁻¹) delineate the urethane bond formation. A minimal intensity peak at 3340–3443 cm⁻¹, which corresponds to free NH-groups, suggests that most of the amide groups are engaged in the formation of hydrogen bonds (Figure S3). This observation is consistent for all examined samples. A pronounced decrease in the peak associated with hydrogen-bonded NH-groups in TPU-2(50/50) relative to TPU-4(100/0) and TPU-1(0/100) indicates a more disordered structure. This diminished microphase separation can be attributed to the incorporation of a PBA/PCL blend as soft segments, which is in good agreement with the SAXS data.

Due to TPU’s multicomponent nature, free -C=O group absorption bands overlap with bands linked with hydrogen-bonded SS and HS, complicating quantitative analysis. Examination of model compounds may provide a solution. Six bands at 1740, 1738(1731), 1721(1729), 1708, 1700, and 1686 cm⁻¹ are identifiable from the second derivatives of FTIR spectra (Figure S4, Table S2). These bands relate to differing interactions of carbonyl groups, and their intensity and position depend on the TPU composition [28,29,30].

For TPU-4(100/0), a shift in υNH, δNH, and υC=O vibrations to 3339, 1531, and 1729 cm⁻¹, respectively, and the emergence of a band at 1684 cm⁻¹, confirms the phase separation (Figure 4, black curve). The phase composition of SS was analyzed using PBA spectra according to our previous study. The second derivatives of the FTIR spectra reveal CH₂ vibrations at 908, 957, 1368, 1398, 1416, and 1461 cm⁻¹ (Table S2), indicating the presence of stable α-crystals of PBA and a mixture of α- and β-phases of PBA after crystallization at 25 and −5 °C, respectively (Figure S5, Table S3), corroborating the WAXS results (Table 4).

The FTIR spectra for TPU-1(0/100), based solely on the PCL segment, display a decrease in intensity and a shift toward low frequencies at 1722 cm⁻¹ of the C=O band, an increase in peak intensity at 1738 cm⁻¹, and the emergence of a pronounced band at 1730 cm⁻¹ (Figure 4, blue curve). The data suggest the presence of free carbonyl groups of HS and hydrogen-bonded carbonyl groups in the mixed phase. The reduced crystallization rate of PCL compared to PBA and the associated increase in soft segment mobility lead to a decrease in dipole–dipole interactions, characteristic of PCL crystals, as we have shown above and in previous work [18]. The low degree of phase separation hinders the formation of hydrogen bonds between urethane groups. A decrease in the crystallization temperature leads to a reduced band intensity at 1731 cm⁻¹ (HS) and heightened band intensity at 1722 cm⁻¹ (SS-SS) and 1684 cm⁻¹ (HS-SS), as evidenced in Figure 4b and Figure S4 and Table S2. This suggests an enhanced hydrogen bonding density within the SS domains, attributable to enhancement in PCL crystallinity and augmented phase separation.

For TPU-2(50/50), a decrease in the peak intensity at 1729 cm⁻¹ and the appearance of a shoulder at 1708 cm⁻¹ indicate the presence of a mixed phase due to intermolecular hydrogen bonding between the amide group of HS and the carbonyl group of SS (Figure 4, red curve). We assume that the non-separated mixed phase can be attributed to the slow crystallization rate of PCL and PBA. These results are consistent with DSC data on the correlation of the degree of crystallinity with the composition of the polyols (Table 3). The change in the absorption intensity of the -NH and -C=O groups represents that the hydrogen bonding efficiency depends on the nature and crystallization kinetics of the SS and on the content of the crystalline phase. Crystallization at −5 °C leads to a decrease in peak at 1738 cm⁻¹ and an increase in peak intensity at 1729 cm⁻¹, indicating a decrease in the mixed phase content due to additional dipole–dipole interactions in the crystalline phase of PCL and PBA. Crystallization at a lower temperature results in a more pronounced phase-separated morphology due to increased hydrogen bonding, contributing to the growth of total crystallinity compared to crystallization at 25 °C. For TPU-3(80/20) and TPU-4(100/0) with high PBA content, the crystallization at low temperature does not show any significant changes (Figure 5b, green and black curves). These results are in good agreement with the DSC data.

Thus, adjusting the crystallization conditions and soft segment content can manage the hydrogen bonding density and phase separation efficiency. These factors influence the rate of formation of independent crystalline phases (PBA and PCL) and the final degree of crystallinity.

3.5. Mechanical Properties of the TPUs

Tensile Properties

The differences in morphology of the TPUs are reflected in their mechanical properties (Figure 5 and Figure S6). One can see that the Young’s modulus of the materials is directly related to the total degree of crystallinity because the crystals play the role of additional physical crosslinks. The maximum Young’s modulus is observed for TPU-4(100/0) crystallized at 25 °C—107 MPa (Figure 5b, black solid line, Figure S6,

Table 5. Mechanical properties of TPU films after 0.5 months of crystallization at 25 °C and −5 °C.

Sample	Young’s Modulus (MPa)	Tensile Strength at Break (MPa)	Elongation at Break (%)	Young’s Modulus (MPa)	Tensile Strength at Break (MPa)	Elongation at Break (%)
	Tc = 25 °C			Tc = −5 °C
TPU-1(0/100)	8.4 ± 0.9	34.3 ± 1.5	1270 ± 40	38.8 ± 0.9	39.5 ± 1.3	1040 ± 20
TPU-2(50/50)	3.6 ± 0.1	23.9 ± 0.2	1295 ± 38	11.5 ± 0.4	32.8 ± 1.5	1140 ± 40
TPU-3(80/20)	25.0 ± 2.1	24.5 ± 0.5	1180 ± 30	24.7 ± 0.8	25.8 ± 1.8	1170 ± 30
TPU-4(100/0)	107 ± 3.7	25.3 ± 0.9	990 ± 60	-	-	-

). The presence of PCL in the TPU drastically reduces the mechanical modulus, which indicates the strong impact of the crystal phase composition on mechanical properties. All the samples crystallized at 25 °C demonstrate similar strength and a certain decrease in the elongation at break with the increase in the PBA content (Table 5).

Crystallization of TPU-1(0/100) at −5 °C leads to appreciable growth of crystallinity resulting in the growth of Young’s modulus to 38.8 MPa and tensile strength at break to 39.5 MPa, respectively (Figure 5a, blue curves, Table 5). The strongest effect of crystallization temperature is revealed for TPU-2(50/50) due to the drastic increase in nucleation and the growth rate of PCL crystals (Figure 5a, red lines), which changes the mechanical behavior from elastomeric to thermoplastic. For TPU-3(80/20) and TPU-4(100/0) with high PBA content, the role of crystallization conditions is rather small (Figure 5b, green and black curves).

The manipulation of synthetic parameters and thermal history can yield polyurethanes with a diverse range of properties, spanning from highly crystalline thermoplastics (TPU-4(100/0)) to soft elastomers (TPU-1(0/100)). For TPUs with dual crystallizable blocks, the degree of crystallinity and rate of crystallization can be adjusted by the mutual influence of the two polyesters during crystal phase formation. The outcomes demonstrate that certain factors, such as post-processing and storage conditions, must be meticulously regulated for the development of adaptive materials. This comprehension will facilitate the acquisition and characterization of novel functional polymers.

4. Conclusions

A series of segmented multi-block thermoplastic polyurethanes was synthesized using poly(butylene adipate) diol, polycaprolactone diol, and their mixture as soft segments, 2,4-toluene diisocyanate and 1,6-hexamethylene diisocyanate as hard segments, and 1,4-butanediol as a chain extender. These polymers develop various phase-separated morphologies, which provide diverse mechanical and thermal properties, as well as different thermal stability.

For TPU-1(0/100) based on a PCL soft block, efficient phase separation of hard and soft segments results in the formation of material with a relatively high degree of crystallinity. Storage of the sample at −5 °C results in a significant increase in the crystallization rate of the PCL domains. Regular crystal formation correlates with enhanced mechanical properties and elevated melting temperatures of PCL crystals.

TPU-2(50/50) with an equimolar blend of PCL and PBA exhibits the pronounced interdependence of soft blocks on phase separation kinetics and crystalline phase development. After two weeks of storage at 25 °C, this sample demonstrates rubber-like behavior and a low degree of crystallinity. FTIR data indicate a substantial fraction of a mixed phase, suggesting restricted phase separation between urethane and polyester segments. A decrease in crystallization temperature notably enhances the nucleation rate of PCL crystals. The emergence of a network of finer PCL crystallites augments the material’s elasticity, with the transition temperature remaining largely unchanged at 40–42 °C.

Increasing the PBA content correspondingly elevates the overall crystallinity and enhances phase separation. For TPU-3(80/20), which crystallizes at 25 °C, PBA crystals predominantly constitute the crystalline fraction, exhibiting a notably elevated melting temperature of 49 °C. Annealing this sample at a low temperature facilitates the formation of an independent fraction of PCL crystals, introducing an additional switching temperature for mechanical properties at 41 °C.

TPU-4(100/0), which is exclusively based on the PBA soft segment, displays the most pronounced phase separation at ambient temperature, as evidenced by FTIR analysis. The high crystallization rate and formation of large domains of soft and hard segments account for its elevated crystallinity and thermoplastic mechanical behavior with an excellent TPU modulus of 107 MPa. When subjected to crystallization at −5 °C, there is a notable augmentation in the overall crystallinity. This can be attributed to the emergence of a metastable β-modification, which subsequently leads to an enhanced melting temperature for the material.

Consequently, the combination of differential scanning calorimetry, wide-angle X-ray scattering, Fourier-transform infrared spectroscopy, and mechanical tests provide insights into the effect of the soft segment composition and crystallization conditions on the phase-separated structure, crystallization kinetics, and mechanical properties of TPUs. Importantly, for these materials, both chemical composition and storage temperature act as independent variables influencing the degree of crystallinity, crystallization rate, and melting temperature. These parameters are paramount for the design and fabrication of materials, wherein mechanical properties can be predictably modulated.